Insulators

ABSTRACT

A composite article is formed from yarn, comprising high strength fibres and fusible blocking material, and outer fusible material. The yarn is formed into its required shape and placed in a mould. The outer fusible material is introduced into the mould and its temperature is raised so as to melt the lower-melt temperature blocking fibres of the yarn. Under the action of temperature and pressure in the mould, the blocking material fuses so as to fill any interstices between the strength fibres, and the yarn is enclosed within the outer fusible material. The method is useful for forming electrical insulators of high strength glass fibre in the form of a loop (4) blocked against flow of moisture by polypropylene and enclosed within non-tracking polyethylene electrically insulating material (6).

This application claims the benefit under 35 U.S.C. §120 of the filingdate of international application no. PCT/GB 93/01210, filed Jun. 8,1994, published as WO93/26016 Dec. 23, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite, for example electricallyinsulating, articles, including electrical insulators per se, and inparticular to reinforced polymeric insulators.

Although the invention is preferably concerned with articles thatperform a solely insulating function, and the ensuring description, forconvenience, will relate mainly thereto, it is to be understood that ingeneral the composite, for example, electrically insulating articles mayalso or alternatively perform other functions. Thus, the article may bea surge arrester, which for most of its time acts as an insulator butwhich, on the application of an over-voltage thereto, becomesconducting, in order to divert a surge of electrical power, from alightning strike on associated electrical equipment for example, toearth. Broadly, however, the invention relates to composite articleswhether electrically insulating or conductive.

2. Description of the Prior Art

Electrical insulators for use at voltages in excess of about 1 kV, andtypically at 24 kV, can be formed from (i) porcelain, which has goodelectrical properties but is heavy and brittle, thus having lowresistance to vandalism; (ii) glass fibre rod impregnated with an epoxyresin, which is of lighter weight but generally has inferior electricalperformance to porcelain in outdoor applications, or must beenvironmentally sealed; or (iii) solid polymeric material, whichcombines the advantages of light weight and good electrical performancebut has poorer load bearing properties. Furthermore, polymeric tubingand/or sheds (annular extensions) may be added to porcelain or glassfibre rods, or incorporated into polymeric insulator, for enhancedperformance. Reference is hereby made to GB-A-1 292 276, US-A-4 045 604,GB-A-1 530 994, GB-A-1 530 995 and EP-A-0 253 622 (all to Raychem) andto GB-A-1 313 609 (BICC) for examples of such insulators and components.

One disadvantage of the use of glass (or other) fibre in theconstruction of an insulator is the possibility of moisture wickingalong the fibre and thus providing an electrical short circuit betweenthe terminals, usually one at each end, of the insulator that makeelectrical connection therewith. Such material however can provide highmechanical strength.

SUMMARY OF THE INVENTION

In general, it is an object of the present invention to provide a methodof making a composite article of high strength that contains fibres, insuch a way as to avoid, or at least reduce, the risk of moisture flowingalong the fibres.

It is also an object of the present invention to provide an electricalinsulator that provides the advantages of glass (or other high strength)fibre and of polymeric material, and to do so in a relativelyinexpensive manner, whilst obviating, or at least reducing, the problemof moisture flowing from one end of the insulator to the other.

Thus, in one aspect, the present invention provides a method of making acomposite article, the article comprising yarn enclosed within outerfusible material, wherein

(i) the yarn, comprising a high strength fibre and fusible blockingmaterial, is disposed in a mould, and

(ii) the fusible material is introduced into the mould in a moltenstate, the temperature of the fusible material being at least equal to,and preferably above, the temperature at which the blocking materialfuses, thereby to cause the blocking material to melt and flow betweenand around the strength fibres of the yarn so as completely to enclosethe fibres therewithin, and such that the blocked yarn is completelyenclosed within the outer fusible material.

The term yarn is used to mean a plurality of fibres, which may be mono-or multi- filament fibres, and is considered to be synonymous withstrand, tow, thread or tape.

By "fusible" is meant a material that flows on becoming hot and whicheither cures under the influence of the heat or solidifies on cooling,and thus includes thermoplastic and thermosetting materials.

Although the method is applicable to the manufacture of both insulatingand non-insulating composite materials, for convenience, reference willhereinafter be made to use of the invention for manufacturing insulatingarticles.

The method of the invention thus provides blocking (against moisturetransmission) of the high strength fibres in a single process that alsoforms the enclosing insulating body. The pressure of the insulatingmaterial entering the mould, especially when it is evacuated, assists indriving air out of the interstices between the fibres, and itstemperature causes the blocking material to melt and to flow completelyaround the fibres. It will be appreciated that although the blockingmaterial is expected to be molten at the temperature at which the outerinsulating material flows into the mould, its complete encapsulation bythe latter material ensures that the blocking material remainscompletely around the high strength fibres disposed for exampleuni-directionally or multi-directionally so as to form a tube, rod orloop. When used in electrically insulating applications, such blockingwill thus prevent flow of moisture through the article and also preventelectrical tracking taking place between the inner structural core andthe outer casing material of the article.

Such a manufacturing process allows the article to be made using asingle moulding process at a comparatively low cost.

In one embodiment, there is provided by the method of the invention anelectrical insulator, preferably of elongate configuration, comprising asubstantially closed, preferably elongate, loop of high strength,preferably insulating, yarn of fibres that are blocked against thetransmission of moisture therealong, the loop extending around meansarranged for the attachment of electrical and mechanical connections tothe insulator, and the loop being enclosed, preferably embedded, withininsulating polymeric, and preferably substantially electricallynon-tracking (in accordance with ASTM D2303), material that preferablyforms an outer surface of the insulator.

Multi-filament glass fibres are preferred as the strength member of theyarn, but other high strength fibres, such as carbon, or aramid fibres,for example those sold under the trademark KEVLAR, may also be used.Each multi-filament fibre, as well as containing a plurality of glassfilaments may also contain the blocking material, preferably in stapleor continuous or other fibre form, or in powder form or as a coating onor within the strength fibres. The structure and blocking of suchfibres, which forms part of the present invention, is disclosed inallowed, copending commonly assigned U.S. patent application of Park etal., Ser. No. 08/276,005, filed Jul. 15, 1994, the entire disclosure ofwhich is incorporated herein by this reference. Alternatively, thefibres may be monofilament. In either case, blocking may avoid or reducevoids between filaments of the yarn and a surrounding polymericmaterial, such as the outer coating material of the insulator. Theblocking of the or each filament of the yarn may be enhanced by theprovision of blocking material around the or each filament.

The outer fusible and/or blocking material advantageously comprises athermoplastic or a curable material, preferably polypropylene,polyethylene, silicone, polyamides, polyethyleneteraphthalate,polybutyleneteraphthalate, polyesters, acrylics, ethylene-vinyl-acetatecopolymer, polyacrylonitrile, polymethylmethacrylate or low strengthglass.

The fibrous blocking material, of mono- or multi-filaments, may besubstantially axially aligned with the high strength fibres, may bewrapped helically therearound, or may be interwoven therewith to form atwo- or three-dimensional mat. In the former cases, an elongateinsulator, for example, so produced would have high tensile strength,and in the latter case, an insulator for use in cantilever applicationscould be produced.

Alternatively, the blocking material may be a powder or a coatingdeposited on or within the filaments of the high strength fibres.

The outer electrically insulating material advantageously comprises apolymeric material, preferably of a polyethylene or of a siliconepolymer.

EP-A-0 265 265, and its corresponding patent publications in othercountries, relates to an article incorporating fibres and discloses theblocking of such fibres against moisture (e.g. water or other liquid)wicking therealong, and the entire disclosure of that publication isenclosed herein by virtue of this reference.

In a preferred embodiment for making an insulator for example, the yarnis wound repeatedly, say of the order of two or three hundred times,around two mandrels that are spaced apart, transversely to the directionof winding, at the required separation of the end terminals of theinsulator, forming an elongate loop of rounded rectangular configurationthat is substantially closed, that is to say it is formed from a singleyarn, which may itself comprise many filaments, and thus has only twoexposed yarn ends. The yarn, held under tension between the mandrels, isthen transferred to a mould for forming the polymeric insulatingmaterial therearound to provide the appropriately-shaped outer surfaceof the insulator--typically comprising one or more laterally-extendingpolymeric sheds. Under the higher temperature and pressure of themoulding operation, the blocking materials melts, or at least softens,so as to be urged between and around the glass filaments to formblocking along the path of the loop, at least at one point therealongand preferably throughout its entire length. Upon cooling and/or curingthe blocked fibre cored, polymeric insulated component is removed fromthe mould. The mandrels can now be removed. The mandrels are envisagedto be rigidly secured together during cooling and/or curing to maintaintheir spacing apart. However, particularly when the insulator is to beused as a tension insulator, it may not be necessary for the blockedinsulator to be rigid. It is also envisaged that electrical andmechanical connection end fittings may be provided as part of themandrels, which may or may not be electrically conductive, and wouldthus subsequently remain in position. In the preferred embodimenttherefore, the end fittings can be secured in position in the singleoperation that effects blocking of the fibres and also moulding of theinsulating body of the insulator.

The mandrel or end fittings, as appropriate, which may be formed as arod or tube or other suitable configuration, are advantageously concaveat their outer surfaces, which receive the yarn thereover, to assist inthe lateral retention of the yarn during the winding operation. The endfittings may be of metal, but any other suitable conductive material,such as high strength conductive polymer, may be used. Alternativelythese components may be electrically insulating.

The polymeric material can be chosen to suit the application, but maytypically be high density polyethylene or similar material, preferablycontaining a track-reducing filler, such as alumina trihydrate, toreduce the tendency of forming carbonaceous paths formed, in operation,from the flow of leakage current along the outer surface of theinsulator. Suitable materials are disclosed for example in GB-A-1 337951, GB-A-1 337 952 and GB-A-1 590 723 (Raychem). Other materials, suchas thermosets for example, may also be used. Also, the polymericmaterial and/or the yarn may be cross-linked.

It is thus seen that the fibrous loop is completely encased within, andbonded to, the outer polymeric material, preferably such that theinsulator is substantially void free.

The end fittings, which may be short lengths of metal tube that extendtransversely to the (major axis of the elongate) loop, lie within theloop such that great mechanical strength is provided--typically theinsulator can withstand an axial loading of 70 kN, which is the minimumrequirement for its use as a 24 kV tension insulator.

The end fittings advantageously are so shaped as to spread out anyelectrical or mechanical stress thereon, for example so as to miniraisethe likelihood of fracturing the loop that is wound therearound. To thisend, an oval or a pear shape is preferred to a circular cross-section,and preferably exhibits a low coefficient of friction with respect tothe loop. Surfaces of the end fittings are preferably also rounded so asto avoid any points at which damaging electrical activity can occur.

It is to be understood that the term "insulator" as used herein refersto an article that performs an insulating function. It is envisaged thatsuch an article may not perform an insulating function for the whole ofits lifetime and/or may perform a function that is not solelyinsulating. In this respect, it is envisaged that the article may beassociated with, for example may encompass within its insulatingpolymeric material, other functional components. For example thecomponents may be varistors such as metal oxide components that allowthe article to operate under certain conditions as a surge arrester toprovide protection against lightning strikes or other over-voltagesapplied to associated equipment.

Embodiments of the methods of manufacture of an insulating article, andarticles manufactured thereby, each in accordance with the presentinvention, will now be described, by way of example, with reference tothe accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional plan view of a first insulator;

FIG. 2 is a sectional side elevation of the insulator of FIG. 1;

FIG. 3 is a sectional end view of the insulator of FIG. 1;

FIG. 4 shows diagrammatically a manufacturing method for the insulator;

FIG. 5a and 5b show schematically two alternative embodiments ofsecurement to end fittings;

FIGS. 6 to 8 show partial views of three embodiment of surge arresters;

FIGS. 9 to 11 schematically show sections through further embodiments ofinsulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 3 of the drawings, a pair of metal bobbins 2 areformed from a short length of metal tube whose outer surface, at leaston one side is made concave. The bobbins 2 are held apart at therequired separation of the end fittings of the completed insulator bymeans not shown. A single multi-filament glass fibre hybrid yarncontaining polypropylene blocking material, is wound under tensionaround the bobbins 2, to engage the concave surfaces thereof, forapproximately two hundred turns, to form an elongate loop 4 of roundedrectangular configuration. The resulting configuration is similar tothat of a fan belt extending around two pulleys. The loop 4 comprisesstrength filaments of glass together with blocking filaments ofpolypropylene. Typically, the loop would be of length 200 mm, width 17mm (ie. the length of the concave portion of the tubular metal bobbins2), and a thickness 3 mm.

The loop 4, still under tension, is then transferred to an injectionmould (not shown). The mould is evacuated and heated to remove airtrapped within the yarn, and non-tracking insulating polymeric material6 is injected at a temperature of about 125° C. into the 200° C. mouldat a pressure of about 2000 psi. Under the effects of the hightemperature and pressure of the mould, the blocking filaments of thefibre forming the loop 2 melt and flow so as completely to encase theglass components and to fill the interstices therebetween, thus forminga rigid, moisture-blocked loop, and the polymeric material subsequentlycools and/or cures.

As shown, the polymeric material 6 is arranged to form the outer surfaceof the insulator. To this end the mould is arranged to provide aplurality of sheds 8 of different radial dimensions. From FIG. 2, it canbe seen that the polymeric material extends completely around theoutside of the loop 4 and also over its inner surface. Cut outs 10 areprovided to reduce the weight, and also cost (due to less material 6being used) of the insulator, and also to enhance the impedance to theflow of surface leakage currents therealong.

The metal bobbins 2 remain in position in the finished insulator, andserve electrically to connect the insulator under tension to suitableelectrical equipment.

Although the strength fibres of the cores of the insulator of thepresent invention are shown and discussed as forming a straight loop, itis envisaged that the loop could be formed with a cross over during thefilament winding step. Also, the loop could be twisted, once or aplurality of times, for example to form a tightly twisted configurationprior to insertion into the mould.

The use of simple end fittings, such as the pressed steel ring bobbins2, in the present insulator dispenses with the need for the machined orforged fittings that hitherto have been necessary. With conventionalcrimp fittings for example a close tolerance is needed in order on theone hand to provide a sufficiently high gripping force and on the otherhand to avoid crushing the glass fibres of a fibre rod insulator.

Referring to FIG. 4, a continuous in-line process is depicted allowingcomparatively inexpensive manufacture of part at least of the insulator.

A pair of spaced-apart, elongate metal rod mandrels 20, 22 with acontinuous single glass fibre hybrid yarn 24 having blocking materialassociated therewith wound in helical configuration therearound are fedinto a hopper (not shown) that delivers hot outer insulating polymericmaterial 26. On cooling, the yarn 24 becomes blocked against moisturetransmission therealong and the yarn 24 and rods 20, 22 becomecompletely encapsulated by the polymeric material 26. When cool,discrete blocked insulators can be cut to length by cuts transversely tothe direction A to give an insulator of substantially uniform transversedimension x.

Sheds (not shown) may then be added discretely, or by moulding in place,or by recovering a shedded tubular part therearound, thereby to providean outer configuration as shown in FIG. 1.

In an alternative arrangement for the insulator of the presentinvention, it is envisaged that the insulation of the fibre-wound endfittings may be provided by two materials. An inner, relativelyinexpensive material may be selected to have properties that facilitateblocking of the filaments of the yarn, and a compatible outer materialmay be selected to have good electrical surface properties, such as highresistance to the formation of conductive tracks therealong.

The fibrous loop used in the insulator may alternatively be formed as abraid, weave or knit providing a band, which may be slit to provide twoor more loops of the required transverse dimension. In this embodiment,the continuous yarn would be arranged to extend in the direction inwhich, in use as a tension insulator for example, tension would beapplied to the insulator.

Although only a single insulator is shown in the Figures, it isenvisaged that by appropriate design of the end fittings and orinterconnections, two or more of the insulators could be joined togetherto form a chain. For example, end fittings could be designed so asdirectly to interlock with one another, or a connecting link or otherdevice could be provided. Such an extended insulator would then besuitable for operation at higher voltages.

Referring to FIGS. 5a and 5b, a single yarn loop 30 is shown extendingaround one of two metal eyelets 32, which ultimately will form the endfittings of the insulator. In FIG. 5a, the yarn 30 extends between thetwo eyelets in a twisted configuration, whilst in FIG. 5b, a free end ofthe fibre 30 is secured to an adjacent yarn portion by a crimp 34. Outerpolymeric material is subsequently applied to provide insulation and toproduce and/or enhance the blocking of the yarn, as hereinbeforedescribed.

In FIG. 6 there is shown a stack of five zinc oxide varistor blocks 40sandwiched between two metal terminals 42. A loop of a single yarn 44extends around the blocks 40 and terminals 42 to retain them in axialcompression. Outer polymeric insulation material 46 encapsulates theyarn 44 terminals 42 and blocks 40 completely, thereby to provide ashedded insulator arrangement that acts as a surge arrester having ablocked single yarn strength member 44. If desired, the polymericmaterial may be arranged to provide for venting from the varistor blocks40 of any gas generated during their operation as a surge arrester,whilst maintaining complete encapsulation of the blocked loop 44.

FIG. 7 shows an end portion of a surge arrester before its polymericencapsulation, in which a metal terminal 50 is of semi-cylindricalconfiguration having a central connecting stud 52 that ultimately willproject beyond the polymer. Two closed loops 54 of yarn are wrappedaround respective shoulders of the terminal 50, and extends so aslikewise to engage a corresponding terminal (not shown) at an oppositeend of the arrester. One varistor block 56 is indicated. The loops 54act as strength members to hold a stack of blocks 56 in compressionbetween the two terminals 50.

FIG. 8 shows a cut away plan view of a modified surge arrester, in whichtwo single loops of yarn 60, 62 overlap each other in cruciform shape asthey extend over an end fitting (not shown) of a surge arrester, insimilar manner to the single loop arrangement shown in FIG. 6. Theblocked loops 60, 62 are shown encapsulated in insulating polymericmaterial 64.

It is also envisaged that the insulator of the present invention may beused as a support and/or guide member for additional lines such as powerand telecommunication conductors, or for other wiring.

Referring to FIG. 9, the closed yarn loop 70 is disposed around two endmandrels 72, 74 and blocked and embedded within polymeric insulationmaterial 76 in the manner described hereinbefore. In this embodimenthowever, a further hole is defined by an intermediate mandrel 78. Themandrels 72, 74 and 78 may, but need not be electrically conductive.Supply lines may then be arranged to pass through respective ones of themandrels. It will be appreciated that the apertures 10 in the polymericinsulating material 6 of the embodiment of FIGS. 1 to 3 may also be usedfor this purpose. The mandrels 72, 74, 78 or the apertures 10 are thusseen to support supply lines extending therethrough, and keep themspaced apart and insulated (where necessary) from each other.

FIG. 10 shows a modification of the insulator of FIG. 9, in which twoclosed loops 80, 82 of yarn are employed, in place of the single loop ofFIG. 9, around respective pairs of three mandrels 84, 86, 88. The loopsextend in substantially the same plane as each other.

As a further modification of the embodiment of FIG. 9, the single loop70 may be formed into a figure-of-eight between the mandrels 72 and 74,thus being more closely wound around the intermediate mandrel 78, andachieving a result similar to that of FIG. 10.

FIG. 11 shows an insulator arrangement that is modified so as to includea rigid axial member that allows the insulator to be used in compressionas well as in tension. A ceramic rod 90 has a mandrel 92 secured to eachend thereof. A closed single loop 94 of yarn is then applied around themandrels 92, and outer insulating material 96 applied therearound in themanner hereinbefore described. This insulator thus has tensile strengthprovided by the loop 94 and compressive strength provided by the rod 90,thereby providing a versatile component, for use as an aerial insulatoron power cable systems for example.

It will be appreciated that the use of a closed, block loop of yarn inan insulator arrangement that also functions as a surge arrester, may beinstead of or complementary to the use of other components for retainingthe varistor blocks in position and/or restraining them in the event ofany shattering during an over-voltage.

We claim:
 1. A method of making a composite article, the articlecomprising yarn enclosed within outer fusible material, wherein(i) theyarn, comprising high strength fibers and fusible blocking material, istensioned into a loop that is wound at least one time around twospaced-apart support members arranged so as to maintain the tension inthe loop, (ii) the yarn and support members are disposed in a mold, and(iii) the outer fusible material is introduced into the mold in a moltenstate, the temperature of the outer fusible material being at least tothe temperature at which the blocking material fuses, thereby to causethe blocking material to melt and flow between and around the strengthfibers of the yarn so as completely to enclose the fibers therewithin,thereby to produce a blocked yarn, and such that the blocked yarn iscompletely enclosed within the outer fusible material.
 2. A methodaccording to claim 1, wherein at least one of the outer fusible materialand the blocking material comprises a thermoplastic material.
 3. Amethod according to claim 1, wherein the blocking material comprises amaterial that is cured in the mold on exposure to the temperature of theouter fusible material.
 4. A method according to claim 1, wherein theyarn comprises high strength fibers selected from the group consistingof glass, carbon, and aramid.
 5. A method according to claim 1, whereinthe outer fusible material is electrically insulating and issubstantially non-tracking.
 6. A method according to claim 1, whereinsaid blocking material is in the form of fibers that are wrapped aroundsaid high strength fibers of the yarn.
 7. A method according to claim 1,wherein said blocking material is in the form of fibers that areinterwoven with said high strength fibers of the yarn.
 8. A methodaccording to claim 1, wherein the yarn is disposed uni-directionally ormulti-directionally so as to form a rod or tube.
 9. A method accordingto claim 1, wherein the yarn is substantially non-conductive.
 10. Amethod according to claim 1, wherein the mold is evacuated subsequent tothe disposal therein of the yarn and prior to the introduction of theouter fusible material.
 11. A method according to claim 1, wherein theouter fusible material is introduced into the mold at a temperatureabove the temperature at which the blocking material fuses.
 12. Anelongate electrically insulating article comprising yarn enclosed withinan outer fusible material, which article is made by the steps of:(i)locating yarn, comprising high strength fibers and a fusible blockingmaterial under tension at least one time around two spaced apart supportmembers so as to form a loop, the support members being arranged tomaintain the tension in the loop, (ii) disposing the yarn and thesupport members in a mold, and (iii) introducing an outer fusiblematerial into the mold in a molten state, at a temperature at leastequal to the temperature at which the blocking material fuses, therebyto cause the blocking material to melt and flow between and around thestrength fibers of the yarn so as to completely enclose the fiberstherewithin, thereby to produce a blocked yarn, and such that theblocked yarn is completely enclosed within the outer fusible material.13. An elongate electrically insulating article according to claim 12,containing at least one varistor block, thereby to function also as asurge arrester.